This invention relates generally to a ballast for energizing gaseous discharge lamps and is specifically directed toward a light dimming control system for regulating the output to gaseous discharge lamps provided by a solid state, high frequency ballast.
The typical fluorescent tube employs a gas confined in a sealed cylinder of glass incorporating a heating filament at either end. A voltage applied to the filament causes thermionic emission therefrom in initiating and sustaining an arc across the tube, causing the gas to ionize and produce radiation. This radiation activates the fluorescent coating on the inner surface of the glass tube, producing illumination. Generally, mercury vapor is utilized in a fluorescent tube. The same principle is employed, without the fluorescent effect, in high-intensity discharge (HID) lamps.
A variable dimming capability in gaseous discharge lamps is desirable for providing a range of lighting conditions and to reduce lamp intensity for conserving energy when possible. Since fluorescent and HID lamps are constant voltage drop devices, the brightness of the lamp is varied by controlling the current provided thereto. Since the filament voltage must be maintained at a predetermined level to sustain the arc across the tube, difficulties arise when attempts are made to provide a variable dimming capability for the lamp. Prior art systems generally employ a ballast arrangement having a high leakage reactance capable of providing lamp voltage regulation over a range of input and lamp currents. However, the aforementioned difficulties are encountered particularly where the lamp circuit draws low current, for these circuits are then subject to line voltage fluctuation resulting in excessive flicker in light output.
Recent work has been directed toward the development of solid state ballasts for energizing gaseous discharge lamps. This type of ballast would offer obvious advantages over prior art devices, e.g., increased reliability, reduced expense, and increased energy efficiency. However, various technical and economic factors have contributed to the lack of a commercial solid state, high frequency ballast, or inverter, for gaseous discharge lamps. Nevertheless, advances in this area have been made as evidenced by the co-pending patent application, Ser. No. 194,783, referenced above. The present invention is intended to provide a variable dimming capability in particular for the solid state ballast disclosed therein and for any solid state ballast incorporating the general features thereof.